Astrophysics > High Energy Astrophysical Phenomena

Abstract: Central supermassive black holes of active galactic nuclei host hot plasma
with a temperature of $10^9$ K, namely coronae. Like the Sun, black hole
coronae are theoretically believed to be heated by their magnetic activity,
which have never been observed yet. Here we report the detection of coronal
radio synchrotron emission from two nearby Seyfert galaxies using the Atacama
Large Millimeter/submillimeter Array, the Karl G. Jansky Very Large Array, and
Australia Telescope Compact Array. The coronal magnetic field of both systems
is estimated to be $\sim10$ Gauss on scales of $\sim40$ Schwarzschild radii
from the central black holes. This magnetic field strength is weaker than the
prediction from the magnetically heated corona scenario. We also find that
coronae of Seyferts are composed of both thermal and non-thermal electrons.
This may imply a possible contribution of Seyferts to the cosmic MeV gamma-ray
background radiation.